1. Field of the Invention
The present invention relates to a method for data communication between a base station and at least one transponder by, for example, a high-frequency electromagnetic carrier signal onto which information packets are modulated. Each information packet can have a header section, a middle section, and a trailer section. The header section can be provided in a forward link of a data communication between the base station and the transponders for controlling the data communication. The header section can be used in a return link of a data communication in order to transmit information from a transponder to the base station communicating with this transponder. The invention further relates to a base station and a data communication system.
2. Description of the Background Art
The invention resides in the field of transponder technology and more particularly in the field of contactless communication for the purposes of identification. Although applicable in principle to any desired communication systems, the present invention and the problems it was designed to solve are explained below with respect to so-called RFID communication systems and their applications. RFID stands for “Radio Frequency Identification.” For general background on RFID technology, please refer to the “RFID Handbuch” by Klaus Finkenzeller, third revised edition, 2002, which has been published in English by John Wiley & Sons.
In the case of transponders, an electromagnetic signal sent out by a base station is received and demodulated by the transponder. In this context, a distinction is made between active, semi-passive, and passive transponders, depending on how their energy supply is implemented. In contrast to active transponders, passive transponders have no energy supply of their own, so the energy required in the transponder for demodulation and decoding of the received electromagnetic signal must be extracted from the very electromagnetic signal transmitted by the base station. In addition to this unidirectional transfer of energy, bi-directional data communication between the base station and transponder typically also takes place.
The foundation for bi-directional data transmission between base station and transponder is what is referred to as a communication protocol, which defines control information for the data communication in addition to the data transmission to be transmitted.
A generic RFID communication protocol for a prior art data communication between base station and transponder is described in the unexamined German application [Offenlegungsschrift] DE 101 38 217 A1. According thereto, an information packet to be transmitted by the base station to a transponder has at least a header section, a middle section, and a trailer section. The header section defines the amount of data to be transmitted and its identification. The middle section contains the specific data to be transmitted. In the trailer section, the end of the information packet is communicated to the receiver of the specific data that has been transmitted. The data communication is protected with protection mechanisms such as a CRC protection field or parity bits, for example.
A generic RFID method and system for bi-directional data communication is also the subject matter of the Palomar Project, which was established by the European Commission as part of the IST Program. For further information on the content of the Palomar Project, please refer to the generally accessible publication of the European Commission issued on Jan. 11, 2002, which essentially corresponds to ISO standard 18000-6.
For further background on bi-directional data communication between the base station and the transponder, please also see the unexamined German applications DE 102 04 317 A1, DE 100 50 878 A1 (which corresponds to U.S. Publication No. 20020044595), DE 102 04 346 A1 and the European patent EP 473 569 B1.
In most UHF and microwave based RFID systems or sensor systems, data communication between the base station and the transponder is first initiated by the base station in that the base station transmits a query signal (command, data request) to the various transponders located in the vicinity of the base station. The transponder or transponders participating in the data communication typically react to this query with a response signal (response).
The method known as the backscatter technique is one method used for data transmission from a base station to a transponder and back by UHF waves and microwaves. In this method, first the base station emits high-frequency electromagnetic carrier signals, which are received and processed by the transmitting and receiving device in the transponder. In addition, the received signals are modulated with a customary modulation method and are scattered back using the backscatter cross-section of the transponder's transmit/receive antenna. Typical modulation methods for this purpose are amplitude modulation, phase modulation and amplitude shift keying (ASK) subcarrier modulation of the backscatter signal, in which the frequency and/or the phase position of the sidebands is changed.
The data to be transmitted are coded and transmitted with the aid of symbols, which are defined, for example, by the header section of an information packet. In this context, a symbol is used to define or interpret the value of a character to be transmitted. A symbol is customarily represented with the aid of a time period between two sequential field gaps, or so-called “notches.” With amplitude modulation, such a field gap can be created, for example, by suppressing or blanking the carrier signal; or with double-sideband modulation, it can be created by switching the phase position of the carrier signal. The transponder decodes received information packets on the basis of the symbols contained in the header section, or using the time periods associated with the symbols, in that it determines the value of a transmitted signal by comparing its time period with the predefined time period of a symbol in the header section.
Through the selection of the time interval, or the time periods assigned to the symbols, it is possible to adapt the transmission rate to the transmission conditions within a certain range.
In the unexamined German application DE 102 04 317 A1, a generic method for data transmission is described in which defined points in time, or reference times, are defined in the header section. The carrier wave is only examined at these predefined points in time. This method permits error correction during the data transmission by means of the change in the modulation state at predefined points in time.
U.S. Pat. No. 5,649,295 describes an RFID communication system in which the return data transmission takes place with the use of a backscatter method. In this system, the base station transmits a first piece of information to at least one transponder of the communication system, the transponder(s) receive the signal transmitted by the base station, decode it, and take the first piece of information from it. The transponders also have a backscatter modulator, which modulates the backscattered signal derived from the transmitted signal by using a second piece of information derived from the first piece of information. The content of the second piece of information in the backscattered signal contains the data rate or the modulation for the signals backscattered by the transponder, for example.
A similar method for data communication between the base station and the transponder in which the base station controls a piece of information for the transponder's operating information, as for example its modulation or data rate, is described in the German patent application number DE 10335003, which corresponds to U.S. Publication No. 20050053024. In this method, the base station transmits an information packet having a header section, a data section and a trailer section. During transmission of the header section of an information packet by the base station, the transponder communicating with this base station transmits, using the backscatter method, operating information of the transponder relating to the processing of data to be received by and/or transmitted by the transponder. The base station receives this transponder operating information and adjusts at least one transmission parameter for further data communication between the base station and the transponder on the basis of this information.
Most UHF and microwave-based RFID systems or sensor systems are designed as master/slave systems. In such master/slave-based RFID systems, the data communication between the base station and the transponder is controlled by the base station. As described in DE 102 04 347 A1, the control takes place through the header section of an information packet. By means of this header section, or rather, the information contained therein, the base station sets operating parameters of the transponder.
However, existing RFID systems are distinguished in particular by the use of an extremely wide variety of transponders, with the differences among the various transponders deriving primarily from their differing functionalities. In particular, asynchronous transponders must generate their own clock frequency by a separate clock generator internal to the transponder.
The problem with this, however, is that the precise clock rate of the transponder is not known to the base station. Furthermore, the base station also does not know what resolution a given transponder is working with. This is especially serious for the demodulation and decoding by the transponder of the signals transmitted by the base station, since such transponders with a very low resolution are not capable of decoding with high reliability the symbols transmitted at a high data transmission rate by the base station, for example. Especially at very high data transmission rates, such transponders, because of their low resolution, find it correspondingly difficult to distinguish the various symbols from one another, for example “0”, “1”, “EOT”. Data communication with such inexpensive, “low cost” transponders is thus comparatively error-prone, particularly when the base station is informing such a transponder by means of the header section that a relatively high data transmission rate is desired, but the transponder does not support this rate because of its structure.